1. Field of the Invention
This invention relates to the production of anhydrous alkali metal dithionites from formates and sulfur dioxide. It specifically relates to production of sodium dithionite in an aqueous methanolic solution in which both sodium formate and sulfur dioxide are dissolved.
2. Description of the Prior Art
Hyposulfites, also termed hydrosulfites and more properly termed dithionites, are in demand as bleaching agents, such as for bleaching groundwood pulps. Zinc dithionite is being replaced by sodium dithionite because of the shortage and increasing cost of zinc dust to produce zinc dithionite and because of ecological objections to disposal of zinc-containing wastes. Sodium dithionite can be produced by electrolytic and borohydride procedures, but the most economical procedures for making a high-quality solid product increasingly use the formate radical as a means for reducing the valence of the sulfur atom in an aqueous methanolic solution within a pressure reactor while incrementally adding an alkaline formate, an alkaline agent, and sulfur dioxide.
This development can be analyzed as comprising two related streams of technology with respect to techniques for introducing the alkaline agent into the reactor: (1) sulfite-based and (2) hydroxyl/carbonate-based. Sulfite-based technology is believed to have originated in 1910 with British Pat No. 11,010 which teaches in two examples the addition of sodium pyrosulfite or sodium bisulfite to an aqueous alcoholic solution of sodium formate and formic acid while adding SO.sub.2 thereto. No yields or productivities are furnished. It next appeared in 1968 as Japanese Pat. No. 7,003/68 which teaches in its Example 5 the addition of an aqueous slurry of sodium sulfite to SO.sub.2 -methanol solution in the reactor, a procedure which creates excessive acidity.
In 1973, British Pat. No. 1,322,250 taught the stagewise addition at 70.degree.-150.degree. C. of 10-75 percent by weight of the total sulfur dioxide to an aqueous methanolic solution containing all of the remaining reactants, followed by adding the remaining SO.sub.2 at 60.degree.-85.degree. C., sodium bisulfite and sodium sulfite being exemplary alkaline agents in Examples 4 and 5.
The information in these five prior art examples is presented in Table I as three ratios based on unity, in addition to concentrations by weight of reactants within the reactor, for the sulfite-based technology. The SO.sub.2 added includes the SO.sub.2 in the alkaline agent.
TABLE I __________________________________________________________________________ Sulfite-based production of Na.sub.2 S.sub.2 O.sub.4 by reduction of SO.sub.2 with formate ion in aqueous methanolic solution CH.sub.3 OH fed Total SO.sub.2 Total SO.sub.2 Total Reactants H.sub.2 O fed H.sub.2 O fed HCOONa Total Reactor Patent No. Example No. (parts) (parts) (equiv.) Contents, Wt. % __________________________________________________________________________ British 2 5.05 0.53 1.92 27.7 11,010 3 4.00 0.52 1.78 16.8 Japanese 7,003/68 5 3.32 0.47 1.08 19.1 British 4 3.76 1.03 1.14 32.0 1,322,250 5 3.76 1.00 1.11 31.2 __________________________________________________________________________
Hydroxyl/carbonate-based technology began in 1933 with U.S. Pat. No. 2,010,615 and continued, more than 30 years later, with a succession of improvements, particularly including U.S. Pat. Nos. 3,411,875; 3,576,598; 3,714,340; 3,718,732; 3,872,221; 3,887,695; 3,897,544; 3,917,807; and 3,927,190; Japanese Pat. Nos. 7003/68 and 2,405/71; and Belgian Pat. No. 698,247. Alternative use of sodium sulfite, bisulfite, and/or metabisulfite is also mentioned in several of these patents, particularly including U.S. Pat. Nos. 3,411,875; 3,897,544; 3,917,807; and 3,927,190.
These improvements generally comprise adding sulfur dioxide-containing methanol and an alkaline agent to an aqueous solution of an alkali metal formate and holding the resulting aqueous methanol solution at a reaction temperature above the dehydration point of the hydrated alkali metal dithionite in order to prevent the formation of crystals having water of crystallization occluded therewithin. The rate of addition must generally correspond to the rate of production of dithionite; if too rapid, the dithionite ion decomposes, thus reducing yield. The rate of addition therefore effectively controls productivity, measurable as weight of pure dithionite per unit of reactor volume per hour.
The information in 14 examples among these prior art patents is presented in Table II as three ratios based on unity for the hydroxide-carbonate based technology, in addition to the concentrations by weight of reactants within the reactor.
TABLE II __________________________________________________________________________ Hydroxide/Carbonate-based production of Na.sub.2 S.sub.2 O.sub.4 by reduction of SO.sub.2 with formate ion in aqueous methanolic solution CH.sub.3 OH fed Total SO.sub.2 Total SO.sub.2 Total Reactants H.sub.2 O fed H.sub.2 O fed HCOONa Total Reactor Patent No. Example No. (parts) (parts) (equiv.) Contents, Wt. % __________________________________________________________________________ U.S. 3,576,598 1 4.32 2.11 2.26 44.5 3,887,695 1 4.32 2.18 1.50 45.0 2 4.32 2.18 1.71 43.8 3,897,544 1 3.00 0.73 1.17 28.4 2 3.00 1.02 1.16 35.1 3 4.00 0.71 1.16 31.3 5 3.00 0.73 1.00 28.4 3,917,807 1 3.84 1.02 1.13 31.6 2 3.84 1.02 1.13 31.6 3 3.84 1.02 1.13 31.6 3,927,190 1 4.47 1.05 2.13 28.6 2 4.25 1.00 2.13 28.0 3 4.72 1.11 2.13 28.8 4 4.25 1.00 2.08 28.6 __________________________________________________________________________
U.S. Pat. No. 3,887,695 discloses a commercially valuable process that is highly advantageous with respect to productivity and simplicity. Its procedure for dissolving a mixture of NaOH and HCOONa in hot water at high temperature and under pressure was adopted in order to obtain a highly concentrated solution in the reactor and because removing water from 73 percent NaOH would require very high temperatures (73 percent NaOH being an eutectic) and removing water from HCOONa would require storage under pressure to keep the material from boiling. It was discovered that the compromise of adding the two together resulted in efficient dissolving of the HCOONa and forming of an aqueous solution having a solids content of 68.5 percent.
However, in order to obtain optimum productivity, it is necessary: (a) to dissolve all solids, by heating a mixture of water, sodium formate, and sodium hydroxide to a temperature (160.degree. C.) that is critical with respect to saturation and freezing-out of the alkali, and then (b) to transfer the very hot, saturated solution from a dissolving vessel to the reactor. For this reason, the process is subject to difficulties in large-scale industrial production because of freeze-ups within supply pipes to the reactors from slight temperature drops.
The hot solution, moreover, is highly corrosive. Hot NaOH requires nickel, while hot formate requires stainless steel. Corrosion of metals by the solution is also harmful to the process, since small amounts of nickel and iron act as poisons. Zirconium tubing and a Teflon-lined tank were found to furnish satisfactory protection. Nevertheless, potential hazards from corrosion of equipment by the hot alkali and spraying of hot alkaline solutions upon operating personnel are always present.
A carbonate-based process is disclosed in the parent application that obviates the hazards and the freeze-up difficulties that are inherent in forming and transferring a sodium hydroxide solution at 160.degree. C., while operating at similar high reactant concentrations, by adding dry sodium carbonate to methanol while an SO.sub.2 -methanol stream and an aqueous sodium formate stream are being fed thereto. It achieves a productivity of 0.68 pounds of pure Na.sub.2 S.sub.2 O.sub.4 per gallon of reactor volume per hour, comparable to the productivity of the process of U.S. Pat. No. 3,887,695.
This carbonate-based process creates a draw-back with respect to evolution of methyl formate because approximately three times as much of this highly volatile compound is produced when using sodium carbonate as the alkali metal compound as compared to using sodium hydroxide. In consequence, additional condenser and scrubber capacity must be added to the system. It is therefore desirable to be able to use a process in which an alkali metal compound is added that produces minimum quantities of methyl formate.
In the hydroxide-based technology of the prior art, it is theoretically possible to obtain 40-50 percent increased productivity by reducing water to a minimum in the reaction. In practice, attempts to do this always resulted in pronounced decomposition of dithionite. Apparently, the reaction rate at 83.degree. C. is simply not great enough to consume that extra formic acid, which therefore causes high acidity and decomposition. A second effect of using the minimum amount of water in the caustic process is that the by-product Na.sub.2 S.sub.2 O.sub.3 tends to precipitate from the filtrate. Because the precipitated Na.sub.2 S.sub.2 O.sub.3 is in the form of very fine needles, it causes the product to be very difficult to filter. It is accordingly highly desirable to determine the exactly optimum amount of water that should be added, preferably expressed as a useful range of ratios of methanol to water.